Various techniques have been developed for the bi-directional transmission of data over twisted pair wire and similar physical medium. Two standards in particular, Asynchronous Digital Subscriber Line (ADSL) and Very High Digital Subscriber Line (VDSL), have demonstrated their beneficial use in such bi-directional transmission systems. ADSL is defined by American National Standard Institute (ANSI) T1.413 and International Telecommunication Union (ITU-T) G.992.1 (G.DMT) and VDSL is defined by ITU-T G.993.1 and ANSI T1.424. These two standards, as well as other similar standards, are commonly referred to collectively as “xDSL.”
ADSL, VDSL and other similar DSL standards operate in a frequency range above the band reserved for plain old telephone system (POTS) transmission. For example, ADSL typically operates at frequencies between 25 kilohertz (kHz) and 1.1 megahertz (mHz). In transmitting data within the prescribed frequency band, these standards implement a Discrete Multitone (DMT) modulation mechanism that transmits data as a plurality of tones in a corresponding plurality of bins (also referred to as channels, sub-bands or carriers), each bin representing a certain portion of the prescribed frequency range. In ADSL, for example, 256 bins, each having a bandwidth of 4.3125 kHz, are used to transmit a 256-tone DMT symbol. The multitone symbol then is demodulated at the receiving end and converted back to digital data.
While providing for relatively high data transfer rates, the use of DMT over physical mediums such as twisted pair wire can be problematic in the presence of noise. Due to their relatively long length in typical xDSL applications (typically hundreds or thousands of meters), twisted pair wire loops may act as antennae by absorbing energy from radio transmissions. This undesirable absorbed radio energy commonly is referred to radio frequency interference (RFI). RFI is particularly problematic in ADSL and VDSL as a number of common radio transmitters, such as amplitude modulation (AM) radio transmitters and amateur radio transmitters (i.e., “ham” radios), operate at frequencies that fall within the frequency bands of ADSL and/or VDSL. Additionally, in DMT-based systems, such as ADSL and VDSL, RFI introduces noise into bins outside the RFI disturber's band due to sidelobe leakage.
Due to undesirable effects of RFI in xDSL systems, a number of techniques have been developed to minimize RFI and/or its effects. One common method is to utilize shielding around the twisted pair wire loop. While reducing RFI to some degree, shielding typically is costly and often does not attenuate RFI to a sufficient degree. In recognition of the shortcomings of using only shielding to eliminate RFI, digital signal processing-based processes have been developed to remove the effects of RFI at the receiver. The following references are representative of conventional digital signal processing-based processes: Brian Wise et al., “Digital Radio Frequency Cancellation for VDSL, T1E.4, December 1997 (commonly referred as the “Weise method”); Frank Sjoberg et al., “Digital RFI Suppression in DMT-based VDSL Systems,” Proceedings of the International Conference on Telecommunications, June 1998 (commonly referred to as the “Sjoberg method); and J. Cioffi et al., “Analog RF Cancellation with SDMT,” T1E1.4, April 1996 (a modification to the Sjoberg method). Generally, these conventional processes are based on approximating the RFI and adjusting the received signal by the RFI approximation. An RFI estimation is made for every DMT symbol using measurements at a number of bins where the xDSL signal is not transmitted and this RFI estimation may then be used to adjust the corresponding DMT symbol. While the above-referenced methods may provide sufficient RFI reduction in certain instances, they have a number of limitations. For one, they are relatively complex computationally and therefore often require extensive storage and computational capacities. These conventional methods also often require more than one tone to approximate the RFI. Further, in instances where the RFI disturber has a bandwidth greater than the bandwidth of the DMT bin (e.g., a 6 kHz disturber bandwidth in an ADSL system using 4.3125 kHz DMT bins), the performance of these conventional methods may be severely limited.
Accordingly, an improved process for RFI reduction in DMT-based data transmission systems would be advantageous.